The invention relates to a state of charge indicator for a battery which is equipped with a liquid electrolyte, having a transparent viewing rod, which has an observation surface at a first end and has a tip, which is adjacent to a cylindrical outer surface and is formed by a conical reflection surface, at a second end, and having a ball cage which is fitted to the second end of the viewing rod and through which the electrolyte can flow, and in which at least one ball guide channel, which is arranged obliquely to the longitudinal direction of the light rod, is formed for at least one ball with a defined relative density, which extends over the height of the tip of the viewing rod and has end-face and lower boundaries as well as an upper boundary wall which is provided with an interruption for the second end of the viewing rod.
State of charge indicators such as these are based on the fact that the relative density of the electrolyte varies as a function of the state of charge. The relative density of the ball which is guided in the ball guide channel is thus matched to the relative density of the electrolyte, so that the ball is lighter than the electrolyte when the battery is correctly charged, and thus floats in the oblique channel. In contrast, when the battery has been discharged, the relative density of the electrolyte falls below that of the ball, so that the ball sinks downward in the ball guide channel.
According to DE 25 11 426 C2, an oblique ball guide channel is provided, which ends at the tip of the viewing rod. When the ball is floating, because the battery is sufficiently charged, the ball is visible at the end of the viewing rod through the observation surface at the first end. Coloring of the ball thus allows a colored indication (for example green) that the state of charge is correct. Furthermore, the reflection surface at the second end of the viewing rod makes it possible to see whether the battery is filled to a sufficient level with the electrolyte. Specifically, if the electrolyte level has fallen to such an extent that the tip of the viewing rod is no longer immersed in the electrolyte, the conical surfaces at the tip of the viewing rod form a boundary surface with air, so that total reflection occurs on the conical surfaces of the tip of the viewing rod, so that the tip appears as a reflective surface, that is to say, it appears bright. If, on the other hand, the electrolyte level is sufficiently high, the material of the viewing rod is chosen such that the coefficients of refraction of the viewing rod and of the electrolyte are of approximately the same magnitude, so that total reflection no longer occurs. The tip of the viewing rod thus produces an optical connection to the (dark) interior of the battery, so that a dark surface appears on the observation surface (if there is no ball). A ball can, of course, be seen only if the electrolyte level is sufficiently high since, on the one hand, the ball cannot float without any electrolyte and, on the other hand, the reflection surfaces at the tip of the viewing rod result in total reflection, that is to say, observation beyond the light rod is no longer possible. Three different states can thus be identified from the observation surface, namely “bright” for an inadequate electrolyte level, “dark” for an adequate electrolyte level when the battery is discharged, and “color of the ball” for an adequate electrolyte level and an adequate state of charge of the battery.
A state of charge indicator of the type mentioned initially is known from EP 1 120 641 A2. In this document, a ball guide channel is provided, which is intended for guiding two balls with different relative densities. The ball guide channel has a length of about three times the diameter of the balls. The two balls are of different colors, which are not the same as the color of the ball cage. When the battery is discharged, the two balls are located at the lower end of the ball guide channel, so that the upper ball is located under the tip of the viewing rod. If sufficient electrolyte is present, the color of the upper ball (for example red) can thus be seen on the observation surface of the viewing rod. This color indicates that the battery is discharged. With a medium state of charge, the upper (lighter) ball floats up, and disappears from the field of view of the tip of the viewing rod to the upper end of the ball guide channel, while the lower ball does not yet float up, that is to say, it remains at the lower end of the ball guide channel, outside the field of view of the viewing rod. If the electrolyte level is sufficient, the color of the housing (for example yellow) can now be seen on the observation surface. If the state of charge of the battery is good, the lower ball also floats up and strikes against the other ball, which has already floated up, as a result of which it is positioned underneath the tip of the viewing rod. The color of the lower ball (for example green) can be seen on the observation surface, as an indication that the state of charge is good.
If, on the other hand, the electrolyte level has dropped, so that it is no longer in contact with the viewing rod, the total reflection on the conical reflection surfaces of the tip of the viewing rod result in a colorless, bright indication in the observation surface, since the light which is incident through the light rod is totally reflected on the conical reflection surface.
With a state of charge indicator such as this, the balls thus move on a contact line of the upper boundary wall of the ball guide channel, which may be designed with any desired cross section, in particular being circular or square, and has to ensure that the balls are guided in a defined manner at the sides. The upper boundary wall of the ball guide channel is interrupted by a recess, into which the tip of the viewing rod projects. The tip, which is in the form of a point, is in this case located approximately on the imaginary extension line of the upper boundary wall. The angle of the conical reflection surfaces is considerably greater than the inclination angle of the oblique ball guide channel. During operation of a state of charge indicator such as this, it is possible for no unique indications to appear because the balls do not float up completely in the desired manner, but remain jammed in the ball guide channel. It is thus even possible for a partially red and partially green indication to be seen in the observation surface. In general, this can be overcome by knocking the viewing rod. However, it is critical if an incorrect indication that is produced as a result of a ball remaining jammed is not identified and is not overcome by shaking or knocking.
It would thus be desirable to provide an indication for a state of charge indicator of the type mentioned initially that it is less susceptible to malfunctions.